Wire carrier program-distribution system



March 9 1926.

o. B. BLACKWELL WIRE CARRIER PROGRA DISTRIBUTIION SYSTEM Filed Sept. 8, 192:

IN VEN TOR Qflfllac/aaell ATTORNEY Patented Mar. 9, 1926.

* UNITED STATES OTTO B. BLACKWELL, OF PLANDOME, NEW YORK, ASSIGNOR TO AMERICAN TELE- PHONE AND TELEGRAPH COMPANY, A CORPORATION OF NEW YORK.

WIRE CARRIER PROGRAM- DISTRIBUTION SYSTEM.

Application filed September 8, 1923. Serial No. 661,642.

To all whom it may concern:

Be it known that I, Orro B. BLACKWELL, residing at Plandome, in the county of Nassau and State of New York,-have invented certain Improvements in WVire Carrier Program-Distribution Systems, of which the following is a specification.

This invention relates to wire pro am distribution circuits and more particuarly to such circuits employing carrier currents for transmitting the distributed information.

In systems of the type above referred to a plurality of different carrier frequencies are employed as carriers and upon each carrier a separate program is modulated so that by the use of suitable selective apparatus the subscriber may choose'any' one of a number of programs, depending upon the number of carriers employed in the system. In order that the carriers with their modulated programs may be superposed on the line the line is connected to bus-bars upon which all of the channels are impressed. In accordance with the present invention, however, it is proposed todivide the carrier frequencies into groups and apply one group of carrier frequencies to one set of bus-bars and another group of frequencies to'another set of bus-bars, and provide means whereby the subscribers line may be switched by an operator to the desired group. The subscriber may then make his own connection to any program withinthe group. The individual carriers of one set are preferably staggered in the frequency range between the individual carriers of the other set. This enables the use of cheaper selective devices upon the part of the subscriber because the selection is between frequencies which are more widely separated than would be the case with systems of the prior art.

The invention may now be more fully understood from reading the following description in connection with the accompanying drawing, in which Flgure 1 illustrates a schematic circuit diagram illustrating the principles of the invention while Figs. 2 and 3 are curves illustrating the characteristics of the filters which may be used in accordanoe'with the present invention.

Referring to Fig. 1, L designates a subscribers line to be used in a wire program system. At the central stationA, the line is arranged to be connected to either of two sets of bus-bars or 11 by means of a suitable switch 12. The program sources are shown at. 8,, S S and 8,, each program source having devices of a type well known in the art for impressing upon characteristic carrier frequencies the program to be transmitted. The sources S and S are connected with bus-bars 10 through the filters F and F while the sources S and S are connected to the bus-bars 11 through the filters F and F,. For purposes of illustration the carrier frequencies characteristic of the sourcesare 10,000 cycles and 30,000 cycles for sources S and S, and 20,000 cycles and 40,000 cycles for sources S and 5,. It will be seen that the carrier frequencies applied to the bus bars 10 are, by this arrangement, twice'as far apart as they would be if two adjacent carrier frequencies, such as 20,000 and 30,000 cycles, for example, should be applied to the same bus-bar, as wouldbe the case if all four sources were. connected in multiple with the same bus-bars. At the subscribers station fourselective devices,as, for example, filters F,, F',, F

and F, are connected in multiple to the sub scribers line L and a subscribers receiving set R is arranged to be connected to the line through any one of the filters by means of suitable switching arrangements- For purposes of illustration the switchingar'range meutscomprise a plug P associated with the subscribers set and jacks J,, J J and J respectively.

In order to more readily understand the advantages of the system reference is made to the curves of Figs. 2 and 3. Only two carrier frequencies will be impressed upon the subscribers line at one time and while four different receiving filters may be provided at the subscribers loop the rate of cut-off of each filter may be made much less sharp thanif all four frequencies were impressed upon the line simultaneously. At the central office since only two filters are connected with the bus-bars and their characteristic fre uencies are wide apart the cutoffs of these lters need not be so sharp.

Fig. 2 shows the characteristic curves of a set of four filters where each filter is of r the Campbell band type. If carrier frequencies of 10,000 and 30,000 cycles only are impressed upon theline it will be obvious from the curves 1 and 3 that the selectivity between the tWo filters which may be employed is much greater than in the case where four frequencies were on the line and separation would have to be made between closely adjacent carrier frequencies. For example, the selectivity as between the filter curves 1 and 2 is obviously much less than that between curves 1 and 3 and hence if all four carrier frequencies were present it would be necessary to provide many addi: tional sections for each filter in order to make the cut-off sharper so that the curves of two adjacent filters'would cross each other at a higher point than is the case with curves 1 and 2. It will be understood, of course, that the sharpness of cut-off of' the filter depends upon the number of sections employed, the cut-offbecoming somewhat sharper for each additional section employed. This means that in order to get a sufficiently sharp cut-off to afford proper selection between the first and second carriers many additional sections of the filter will necessarily have to be added. Upon the other hand, where the selection is only between the first and third frequencies sufficient selectivity between the circuits will be ossible with a much smaller number of sections. Thereby, a saving will be effected in the cost of the filters, both at the subscribers station and at the central office, although the former saving is of much greater importance than the latter.

Fig. 3 illustrates the case in which the filters employed are high pass and low pass filters. impossible to use high pass and low pass filter combinations for choosing between more than two channels unless the present system were employed, as it is only possible to select between two frequenciesat one time by means of a combination of high pass and low pass filters. The cut-off point of the high pass and low pass filters in this type of circuit may, as in the case of the circuit of Fig. 2, be much less sharp where only every.

other frequency is impressed upon the same bus-bar. It will be apparent from a comparison of the points of intersection of the curves 1 and 3 and of curves 2' and 3 that the cut-off of the curves 2' and 3, for

exam le, would have to be much sharper sections may be used in the filters, both at the subscribers station and at the central office.

It would, of course, be absolutely- Itwill be obvious that the general principles herein disclosed may be embodied in many other organizations widely different from those illustrated without departing from the spirit of the invention as defined in the appended claims.

What is claimed is:

1. In a program distribution system,a plurality of program sources having different characteristic carrier frequencies, a plurality of sets of bus-bars, means to simultaneously impress certain of said frequencies upon certain of said bus-bars and means to simultaneously impress other of said frequencies upon other of said bus-bars, a transmission circuit, means to connect said circuit selectively to said bus-bars and means at a subscribers station: to discriminate between the frequencies transmitted over said transmission line from the bus-bars through which it is connected.

2. In a program distribution system, a plurality of carrier sources of different frequencies, a plurality of sets of bus-bars, means to impress certain of said carrier frequencies on one set of bus-bars and other of said carrier frequencies upon another set of bus-bars, the frequencies impressed upon one set of bus-bars being staggered with respect to the frequencies impressed upon the other, a transmission line and means to selectively associate the transmission line with any of said sets of bus-bars and a subscribers st,

tion having selective means arranged to discriminate between the carrier frequencies simultaneously impressed upon one set of bus-bars. Y

3. In a program distribution system, a plurality of carrier sources of different frequencies, a plurality of sets of bus-bars, means to impress certain of said carrier frequencies on one set of bus-bars and other of said carrier frequencies upon another set of bus-bars, the frequencies impressed upon one set of bus-bars being staggered with respectto the frequencies impressed upon the other, a transmission line, means to selectlvelyassociate the transmission line with any of said sets of bus-bars and a sub scribers station having selective means arranged to discriminate between the car.- rier frequencies simultaneously impressed upon one set of bus-bars, said selective means being designed to give insufficient selectivity between closely adjacent carrier frequencies impressed upon different busbars. 7

4. In a program distribution system, a plurality of carrier frequency sources, a plurality of sets ofbus-bars, means to impress two of said carrier frequencies upon one set of bus-bars and twoother of said carrier frequencies upon another set of bus-bars, the carrier frequencies impressed upon one set of bus-bars being staggered with respect to the other, a transmission line arranged to impressed upon any set of bus-bars to which" be selectively connected to anyone of said the line is connected. sets of bus-bars and a subscribers station In testimony whereof, I have signecl'my 10 associated with said line, said subscribers name to this specification this 5th day of 5 station comprising high pass and low pass September, 1923.

filters for selectively transmitting into separate channels the two carrier frequencies OTTO B. BLACKWELL. 

